Posted
by
kdawson
on Tuesday September 15, 2009 @06:05PM
from the galaxy's-your-oyster dept.

KentuckyFC writes "Gravitational waves squash and stretch space as they travel through the universe. Current attempts to spot them involve monitoring a region of space several kilometers across on Earth for the telltale signs of this squeezing. These experiments have so far seen nothing. But by monitoring an array of pulsars throughout the galaxy, astronomers should be able to see the effects of gravitational waves passing by. They say such an array of pulsars should effectively shimmer as the gravitational waves wash over it, like a grid of buoys bobbing on the ocean. That'll create an observatory that is effectively the size of the entire galaxy. These observations should be capable of monitoring how galaxies and supermassive black holes evolve together, and shed light on the physics of the early universe. Best of all, the next generation of radio-telescope arrays should be capable of making these observations at a cost of around $66 million over ten years. That's a small fraction of the hundreds of millions that Earth-based observatories have already cost."

Detecting or not detecting gravitational waves validates* or invalidates part of Einstein's theory of general relativity. That's a pretty big deal. It means that we have found the first flaw in a theory whitstanding constant attacks on it since 1915 if we would not find gravitational waves.

*take "validates" in this context to mean that there is no experiment or information in disagreement with the theory, therefor going by science's falsification requirement, science considers the theory to be currently valid.

I use them to make binary neutron stars swirl into each other and explode. They remove energy from the system so you can pack those stars in there real good... and rapidly spinning black holes make great gifts for the kids.

Just wanted to point out that the pulsar timing array approach will cover a completely different frequency range (~ 10^-9 to 10^-7 Hz) to existing ground-based detectors (LIGO, Virgo and friends), which operate in the 10^1 to 10^4 Hz range. In between are projects like LISA (http://lisa.jpl.nasa.gov/).

The different frequency ranges mean different astrophysical sources of gravitational waves; generally speaking, the more massive the system, the lower the GW frequency. LISA, for instance, would see the radiation produced by the supermassive black holes at the centres of galaxies, while the other detectors would be targetting much smaller systems.

Thank you for your interpretation of the meaning behind today's article. It was a revelation for me to consider gravity waves as an analogue of electromagnetic waves.
On the The North American Nanohertz Observatory for Gravitational Waves web site [arxiv.org] there is more information. They say "The timing precision of today's best measured pulsars is less than 100 ns. With improved instrumentation and signal-to-noise it is widely believed that the next decade could see a pulsar timing network of 100 pulsars each with better than 100 ns timing precision." I thought it interesting that they only get 100. Then if you did a long term integration of these signals, you may get down to pico-second timing. Such a timing base may be used to correct atomic clocks in GPS satellites and have many other uses. This is all just pure speculation by a non-physicist, so take it with a grain of salt.

The good thing is that the pulsars which glitch are the young ones (hundreds to millions of years old). The pulsars that we are using for NANOGrav are millisecond pulsars which are hundreds of millions or billions of years old, have much smaller magnetic fields than young pulsars, and basically never glitch. They are extremely stable rotators -- much better than normal pulsars.

If the pulsars under observation are, say, 100-1000 light years apart, then the time necessary to notice a gravitational wave perturbation would seem to be on the order of 100-1000 years, respectively.

IOW, because gravitational waves travel at light speed (general theory of relativity), then a "stretch and squeeze" at one pulsar would reach the more distant pulsar many years later. The observed delay is of course a function of the distance between the pulsars, the angle of the wave and the angle of them to earth.

OTOH, a gravitational wave train with a wavelength much shorter the the distance between the pulsars could also be observed if a lot of pulsars were involved -- and if the observation period was at least one cycle. The 10^-9 frequency mentioned equates to a 31.7 year period.

If there are no gravitational waves to be found? If we search the entire spectrum, and we don't find any, then I assume that falsifies the grav-wave theory (and the entire Honorverse). At that point, what is the next step/theory? In a related note, does gravity pull, or push? I think I remember reading somewhere that Einstein said gravity pushed, rather than pulled.

I think that is an excellent question. It's the classic divide between Einstein and Bohr. For Einstein, gravity is geometric, for Bohr et al, it is a product of Stuff and Stuff exists as particles, waves, and/or both.

If they don't find gravity waves in this attempt, I would suspect the following to happen:

A: One bunch, the Einsteins of the lot will say "Well, I toldja so..."

B: The Quantum types will simple demand more money for an even bigger test that will look at clusters of Galaxies or some such co

If gravity waves didn't exist, you'd have to find some other explanation for PSR B1913+16 [wikipedia.org], which is a pulsar in orbit around another star. The pulsar and its companion are spiraling in together, losing energy in exact agreement with the phenomenon of gravitational radiation predicted by General Relativity. This binary pulsar system has been hailed as sufficiently convincing indirect evidence for the existence of gravity waves that Russell Alan Hulse and Joseph Hooton Taylor Jr. were awarded the 1993 Nobel Prize in Physics for its discovery.

No, it doesn't seem that the existence of gravitational waves is in any question here. The only thing is that there might be much yet we don't understand about gravity that is stifling our ability to observe them directly. It's obvious that General Relativity is far from being the final word on gravitation.

Technically speaking, no. They squash and stretch space by definition. If they don't exist, space obviously won't be squashed and stretched by them, but that won't change their definition. They just won't exist.
It's like saying "a unicorn has wings". The fact that it allegedly doesn't exist doesn't mean it doesn't have wings when someone draws one.

No, the article is making a claim about space, which is real. It would be like me saying "Unicorns are eating my lawn," which is obviously not true. What I should say is "Unicorns might eat my lawn, if they exist."

Perhaps the summary author assumed a level of reading comprehension. It might be presumptuous if we knew there were gravitational waves, but not what their behavior was. However, we know their behavior, but not if they exist.

Gravitational waves are found as a 'natural solution' from Einstein's General Relativity, more or less like electro-magnetic waves are a solution to Maxwell's equations. Since GR so far seems to be solid as a rock (supported by many experiments [wikipedia.org]), there is little doubt among theoreticians that GWs exist. Moreover, a binary system containing a pulsar observed by Hulse and Taylor is spinning down at exactly the rate that you would expect if the system loses energy by gravitational waves. This is not a direct o

So, stop me if I'm way off base, but might it be impossible to detect gravity waves? If a gravity wave is a change in the gravitational constant of a finite space, then wouldn't that affect the mass, and the space-time qualities of a sensor within that space, rendering its observations relative, and useless?

A gravity wave, as derived from the theory of relativity, doesn't specify that the gravitational constant would oscillate - simply that the shifting of large masses, like co-orbital black holes and such, will distort spacetime in wavelike manner. Those perturbations of spacetime would travel from their origin outward at the speed of light.

It's best to think of it in terms of the bowling-ball-on-a-rubber-sheet analogy of space-time. If you take a large mass like a bowling ball and set it in the middle of a large rubber sheet, it will depress deeply nearby and taper off the further away from it you go on that sheet. If that bowling ball magically disappeared, there would be a wave that travelled across that sheet as well as if you had 2 bowling balls spinning around each other.

The way we've been trying to detect gravity waves so far (LIGO) uses lasers set up at right angles so if space were to compress or stretch in one dimension, the beams the were previously in phase would shift apart. This can detect a stretching of spacetime equal to a fraction of the wavelength of light used in the lasers.

In actuality, it is the change in the behavior of spacetime that lets us measure in that manner, but if the wave were to stretch spacetime in all dimensions, LIGO couldn't work. Hope that explains it.

Like I mentioned in the last sentence, it relies on the expectation that a gravity wave passing through an area would stretch one dimension of space while contracting another perpendicular to it.If it causes all dimensions (including time) to expand and contract simultaneously, it can't work.

Of course, I have to defer my understanding of gravity waves to those who study this stuff for a living and have experimentally verified a large body of the predictions made by general relativity.

A gravity wave will change the distance to objects at right angles to its direction of propagation. This effect is biggest when the distance to an object is order the wavelength of the wave, or longer. (Since they travel at the speed of light, the relation between wavelength and frequency is the same as for light.) Likewise, the sensitivity is biggest when the period of the wave is between the frequency of measurement and the total duration of observations. So, pulsars are sensitive to waves with periods be

DarrenBaker, a gravity wave is not a change in the gravitational constant; it is a deformation of the space-time fabric itself. So it doesn't change the gravitational (attractive) force between masses but simply moves the "fabric" on which they lie.

Imagine a stretchy, rubber fabric that you pull/push or move upward/downward from one side such that a wave propagates through. Then two masses lying on this fabric, link ping pong balls that you would stick on, would move closer/further apart. That's basically

The really interesting thing is that General Relativity predicts two and only two polarizations, while other theories (that cannot be distinguished from G.R. in the usual solar system tests) have more polarizations. If and when we get a good, high SNR, detection of gravitational radiation, a profound test of gravity should follow in short order.

Bear in mind that there's a big difference between a theory which cannot be tested in principle, and one which cannot currently be tested in practice due to limited technology. Actually, it's not even appropriate to use the word "theory" in this context. In the latter case, it's a hypothesis. In the former, it's metaphysics.

He might not have thought about, but there's actually a fairly simply way to measure the size of an atom.

Take an oil drop and measure its weight. Then drop it on the surface of a very still lake, and leave it to spread it out. It will spread to be about 1 atom thick. Then you can simply look to see how big it spread out, divide by the volume by the area, and the result is the size of the atom.

This gives a result to within an order, which amazing given its simplicity.

Why can't our great minds that are exploring the Galaxy construct their Observatories on the Moon? Maybe on both Poles? The clarity of their images, I believe, would be fairly difficult to equal across any collections of arrays on Earth.

What do you mean finding absolutely nothing? They just ruled out the higher end of the spectrum for gravitational waves. They learned a lot in building very precisely calibrated instruments to do the gravitational wave detection. They continue to lower the detection threshold.

Judging by his links to thunderbolts.info, what I think he means is "I'm a crazy idiot who doesn't understand anything, and think this is a sound foundation to question the work of scientists everywhere. Solar wind is caused by an electric field! What do you mean it's a plasma with equal amounts of positive and negative charges, and a field can't move opposite charges in the same direction? No really, I have no idea what you're talking about because I never too physics in school! But my theories are right anyway!"

> They just ruled out the higher end of the spectrum for gravitational waves.

No. They failed to detect high-frequency gravitational radiation above a certain level. Conventional theory predicts that the radiation they failed to detect should be fairly rare, so the result tends to confirm the established theory while leaving the proponents of some alternative theories with some explaining to do.

The problem with all this is that the original claims for the LIGO detector were that it would detect something... Remember, LIGO's predecessor was a huge iridium bar that also detected nothing. Furthermore, LIGO was significantly improved during its operation, and yet it still found nothing. Now the scientists involved claim they never expected it to find anything? Sounds like the multibillion we have poured into the cure for cancer... still haven't got that! Take another several billion for the next 10

If gravity waves distort space-time, then how can you extricate the observer from her own deformable frame of reference long enough to make a measurement? Shouldn't there be a quantum effect, like teleportation, if a distortion is detected? Since the effect would probably be small, it would probably show up as weird stuff like unexplained cold or miniscule loss of mass in a reference object.

The lack of positive results in gravitational wave detection and the Higgs search reminds me of the Michelson-Morley experiment. Sure I know that we're only scraping the bottom/top of the possible ranges for these phenomena, but I wonder if we aren't just killing time until the next Einstein comes along to explain that there is no luminous aether.

This is precisely this type of condescending, we-are-am-smater-than-you attitude that turns people off on science and scientists. Maybe physicists should concentrate on the foundational issues (e.g., the true nature of motion) first before they go chasing after gravity waves. You folks are not as smart as you think you are.

But of course you are as smart as you think, which is smarter than every other physicist alive or dead (while so pointedly stating that you aren't one), so this is the kind of condescending attitude we need. LOL.

Did you know that over 90% of physicists believe that matter can move in spacetime even though it is known that spacetime is frozen from the infinite past to the infinite future?

Did you know that most physicists believe that moving bodies remain in motion for no reason at all, as if by magic?

They also believe that bodies at rest remain at rest for no reason at all, as if by magic! This is no more mysterious.

Well, magic, and that and for it to do otherwise in either case would require an expenditure of energy and a transfer of momentum.

I'll admit, I bit and read the blog, and it was highly amusing. It was very humorous reading about how you agree with Aristotle* that there must be a "force" to make an object move at a constant velocity, and the object should instantly stop as soon as that "force" is removed. And therefore there must be "energy" around us to make this happen. As if "force" and "energy" are vague, mysterious entities, like a sci-fi writer referring to a "mysterious force" or "a being of pure energy".

But actually, force is a change in momentum. If there was a net force acting on a moving object, it would accelerate (or decelerate). If there's no force on an object, it can't accelerate or decelerate, i.e. its velocity must be constant. If the speed of the object changes, then there was a transfer of energy. Energy, by the way, is the principle Newton was looking for. It's the transfer and storage of energy in various forms that explains how objects can begin moving, and continue moving. Conservation of energy was formulated not too long after Newton's conservation of momentum and fills in what Newton couldn't.

The problem with upending physics is that you have to understand it first. This has been the case for all the great physicists, and it's the case today. And you don't understand causality. An object changing its speed, going from motion to no motion, is the effect, and for this to happen there must be a measurable cause, specifically a transfer of momentum and energy. So, please, Conservation of Energy demands an answer: in the absence of any outside force, why would an object moving at a constant velocity stop?

* Great thinker but lousy physicist -- thought his ideas were so good they didn't need testing**! Must be why you're drawn to him.;)

** Maybe if he had, he would have realized that he was close but off on his idea of an object's natural state in the absence of interference being one of rest, when it's really one of constancy, and we would have Aristotle's Laws of Motion.

The guy you're responding to, tjstork, is an idiot, not worth your time. He's also a conservative, but I repeate myself. The only reason it's relevant is that his opinions come from his ideology. In his mind, you are already wrong because you like science, and science is paid for in large part by public dollars. This makes science the enemy to him.

He'll stick to his scientifically ignorant position, and you will fail to educate him.

Gravitational waves are a consequence of general relativity, so IF gravitational waves don't exist then GR is at least partly wrong. That's a bit stronger than believing in Santa Claus I'd think

Well, if GR was wrong on that score, don't you think physics would suddenly get a lot more interesting? I mean, seriously, its the prospect of Santa Claus popping up and gravity waves not being there that really, fundamentally, the human force that drives science. People want to be surprised by the experiments that

No, you really, really, don't get it. It's not like someone one day decided there are gravity waves, and conned people into spending millions on tests for them.

Eintein's theory of General Relativity (GR) predicts that gravity waves exist, and GR has already made several other verified predictions. It's a bit like a boat in the water. What we've verified with GR already is that the boat displaces water, this is the distortion that objects with mass cause to occur on spacetime. Gravity waves would be the w

Eintein's theory of General Relativity (GR) predicts that gravity waves exist, and GR has already made several other verified predictions. It's a bit like a boat in the water. What we've verified with GR...

No, but my point is that every breakthrough in physics came through because people were ho hum and looking through some theory where they expected to find a result, and didn't. Once upon a time people thought Newtonian mechanics was all there was. We think 100 years of Einstein (wow!), is a long time, b

So you want all experiments to show something contra to the hypothesis, which suggests some new hypothesis? That's what it sounds like. What do you want to happen when they try to test the secondary hypothesis? Do you want that experiment to fail and sugest something else? If no hypothesis are proven, what's the point of experimenting?

I'm really starting to hate the Santa Claus metaphor. There really was a historical person, Nicholas of Cusa, who was Bishop Nicholas in life but became known after his death as Saint Nicolas. That title got shortened and linguistically shifted to Santa Claus.* So technically, there was a Santa Claus, and its just some of the claims made, like his continuing to live today, having flying reindeer or residing at the north pole, that are contra-factuals. Some of the claims, such as his giving a great deal to m

Saint Nicholas is a historical person who has simply had more myths and legends attach themselves to him than has George Washington, also still a historical person despite the Cherry Tree and Dollar across the Potommac stories.

The myth of Santa Claus has taken such a scale that, though Saint Nicholas might be a historical person, it's fair to say that Santa Claus is a separate entity, a myth based on a historical figure.

I guess when you look for evidence of something and find absolutely nothing, it's okay not to abandon the theoretical reasons why you looked for it in the first place.

That's correct. Lack of evidence isn't enough to disprove a theory; what you need is evidence that directly contradicts the theory. In the case of gravity waves, it might be observation of an event that should produce detectable gravity waves, combined with our not detecting them.

And, while I'm at it, I'd like to point out that what Popper taught us was that a theory was useless unless there's a way to falsify it, at least in theory. If you can find a way to show that any conceivable experimental results can be viewed as confirming the theory, it's useless because it can't be tested. In the case of gravity waves, they're but one of many things predicted by General Relativity, and one of the few that's not been observed as yet.

LIGO and Pulsars set limits (or could detect) gravitational waves in very different parts of their frequency spectra - periods of milli seconds versus periods of months. The sources are different, the detection physics is different, etc. It's certainly worth trying both.

Also, none of the existing detectors are good enough that you can say for certain that there are known or likely astrophysical sources bright enough that they should see them. You can't talk about falsifiability until you cross that threshold, which I would expect to see happen in a decade or so.

The theoretical reasons, in this case, include General Relativity, as your first link points out. That's passed a lot of other tests, and dropping it would take some big reasons. Trouble confirming just one of many predictions? Interesting, but not excuse enough to abandon a highly successful theory, not at all. Get a competing theory that has substantially more predictive power, and makes substantially fewer untestable claims, and scientists will generally switch, but none of the electric universe models a

(Unless you can prove scientifically that science is the best way of determining objective truth, and neither we, nor any hypothetical beings living anywhere else in the universe, no matter how powerful their minds are, can ever invent anything better than the scientific method.).

Popper's falsification was a philosophical concept, and might be falsifiable by logic or philosophical debate, but it's not part of science itself, any more than the claim that sc

A human landing on mars gives us pretty pictures and a bunch of cozy, warm feelings.

Understanding fundamental physics (and mathematics) gave us the computer age along with keeping Moore's "law" working for the past 40 years. What did physics ever give to you? Pretty much every major engineering invention since 1950 depends on it in some way or other.

Understanding fundamental physics (and mathematics) gave us the computer age along with keeping Moore's "law" working for the past 40 years. What did physics ever give to you? Pretty much every major engineering invention since 1950 depends on it in some way or other.

The sad thing is that I've met plenty of computer geeks who basically say that physics is useless. They then go back to their beloved computers without realizing the tragic irony of what they just said. That's the pathetic thing about compute

The sad thing is that I've met plenty of computer geeks who basically say that physics is useless. They then go back to their beloved computers without realizing the tragic irony of what they just said.

Still, you're making that remark using a web browser running on top of a software stack made up of at least a multi-tasking OS kernel, a dynamic linker and an assortment of userspace libraries, written in various high-level programming languages with optimising compilers. It's not as if the transistors came up with all that by themselves.

Physics in itself is important, there's just no need for most people to be physicists.

Since when does everything science accomplish have to have immediate material benefit to humanity? Science is the advancement of Homo sapien knowledge about the universe. If you're going to complain about spending money complain about throwing trillions of dollars at the people who brought down the economy. Some people need to grow up.

He's 15 years old. It's much easier for him to understand and critique something that has been summed up than to spend the time and critical thought necessary to understand our present economy. Let's give the little bastard^Wwhippersnapper a break, he's trying.

> Since when does everything science accomplish have to have immediate material benefit to humanity?

To play devil's advocate... it's not so much a matter of "what science accomplishes" as "who pays for it". One is entitled to some sort of opinion when one is paying for it, via tax dollars.

It is, as you point out, shortsighted to assume that all science must have an immediate benefit to be worth it. But it is worth considering whether money spent on basic research might be better spent elsewhere.

Finding gravitational waves isn't going to help 99.99% of the population.

How about you look at this this way instead:

There is a lot of money going around to try to help a flailing economy. Why should that money go ONLY to those who have been bad at their business? Automakers that don't make the right cars? Banks that don't have solid lending strategies? Why NOT give some of that money that's all going to the same economy to scientists who quietly go about their business and get things done?

Hang on, even better. Look at all that money being spent on womens hadbags. It's billions upon billions. What about makeup or alternative medicine with evidence that it does not work. We are now into the trillions...

So I propose before you take a hit on science, maybe you should look at so much other usless spending and save a few kids from hunger

I know it's counter intuitive, and back when I was about your age (that's right, I'm patronising you. Feels good man.) I myself didn't get it, but basic science (the kind that seems like useless theoretical dicking around like that gravitational waves thing) is a sort of long term investment, and a great kind of investment, as you can get several times your investment back.

Think about it, what good was nuclear research in the 19th century? Yet a few decades later they probably kept us safe from an all out

"Umm...actually finding gravitational waves would help 100% of the population."

Finding gravity waves is the key to finding anti-gravity waves. You see, every particle has its anti-particle. The are electrons and positrons. The are protons and anti-protons. There are top quarks and there are bottom quarks. There are charming quarks and there are boring quarks. There are gluons and anti-gluons. Just imagine what a beam of these would do to your enemy. Leaves nothing but quarks flying in all directions! Not a

No it isn't, you fool. There's no Grand Scientist Bureaucracy where they dole out what every scientist is going to work on. There are many different fields and many different scientists who would like to work on many different things. Damn false dichotomies with you people, get a grip on reality for IPU sake.

Pulsars are your measuring instruments. They feed back their measurements as a periodic electromagnetic signal. Since the period, per pulsar, is standard any deviation could indicate a measurement to be researched. Pulsars are distributed roughly equally throughout the galaxy, so in theory we can use the entire galaxy as our observatory.

I agree that we should be funding the tracking of NEOs more, but remember that the money for these two projects isn't coming from the same pool. One is a NASA sub-project, the other is an international project conducted by a variety of observatories and funded by a variety of organizations. So it's not as simple as "do this instead of that".

The speed of light is actually the speed of energy, so if gravity waves are energy, then it will be at or slightly below the speed of light. Otherwise, if it's matter-based, then its maximum speed would be about the speed of sound. I believe.

I seem to recall an experimental observation in the last few years involving Jupiter, through which they verified with about 90% certainty that the speed at which gravity propagates through space/time is equal to the speed of light.

Remeber that mass causes a curvature in spacetime ( General Relativity ), so inside the event horizion, spacetime is curved in on itself so much that light cannot escape, as all possible directions inside the event horizion point towards the center. While a 'calculated' escape velocity inside the event horizion may turn out to be a speed higher than lightspeed, no matter can ever reach that speed.

As far as we can tell, gravity moves at the speed of light, so therefore gravitational waves will